1. Field of the Invention
The present invention relates generally to the bonding of rubber to epoxy resin materials and composite matrices thereof. The present invention is particularly suitable for the bonding of rubber, whether vulcanized or unvulcanized, to curable epoxy resin materials or composite matrices including curable epoxy resins therein. The present invention is especially suitable for, but not limited to, the bonding of unvulcanized synthetic rubber material to an epoxy resin based composite matrix including fibrous material to form certain components of rocket motors constructed by the aerospace industry.
2. State of the Art
Certain components of rocket motors are often constructed by laying down a composite material, or matrix of materials, usually having strength-enhancing fibers embedded in a curable epoxy resin, onto a flame-resistant and, to a certain extents thermally insulative rubber substrate. The rubber substrate and the composite matrix substrate being co-cured at an elevated temperature at a preselected pressure ideally provide a final component consisting of the rubber substrate being securely bonded to the composite matrix substrate.
An exemplary component of a rocket motor constructed in such manner is the motor case assembly which forms the main portion of the rocket motor. Typically, motor cases are constructed to have an epoxy composite exterior and a flame-resistant rubber material lining the interior of the case. In many rocket motor designs, the motor case itself is the primary portion of the rocket motor wherein the epoxy composite exterior forms a rigid skin and propellant fuel is bonded or cast onto the rubber material lining installed within the interior of the rigid skin. In addition to having suitable flame-resistant properties, motor cases must be made as lightweight and as structurally rigid as is feasible because the cases can be quite large in larger motors. Thus, constructing rocket motor cases from epoxy composite materials lined with a flame-resistant rubber material is a common practice within the art due to composite materials being able to provide all of the requisite characteristics needed for rocket motor cases while the rubber material lining provides the needed flame resistance as well as provides a suitable surface to which propellant fuel can be bonded.
In constructing thermally insulated rocket motor cases, an unvulcanized synthetic rubber material such as ethylene propylene diene monomer (EPDM) rubber compound, available from a number of commercial rubber mixing facilities, including the Burke Rubber Company for example, islinitially formed about a generally cylindrically-shaped mandrel, or tooling form, in order to provide the inner size and configuration which the motor case is to have. The use of EPDM rubber material is desired for its flame resistance and thermally insulative properties as well as its low specific gravity. In other words, EPDM rubber material is very lightweight when compared to other commercially available rubber materials. Usually EPDM rubber material is first formed about the mandrel to a desired thickness. Then, typically a composite epoxy resin matrix containing carbon, graphite, boron, aramids, or other high-strength fibrous material, in the form of fibers, tows, strands, or tapes for example, immersed in, or prepregnated with, epoxy resin is hand-laid on or wound about the outer surface of the EPDM rubber by specially designed machines to provide a motor case assembly having an inner layer or lining of rubber material and a composite outer shell. Epoxy resins particularly suitable for forming the base material of the motor case""s composite outer shell are typically aromatic diglycidyl ethers of bisphenol A or F variety and include exemplary widely used epoxy resins such as: EPON 828 and 826 from Shell Chemical Company; DER 332, 383, and 661 from Dow Chemical Company; and LY 9703 from Ciba-Geigy Corporation.
European Patent Application 0 486 044 A2 filed Nov. 15, 1991, entitled Damage Tolerant Composites Containing Infusible Particles, listing Hercules Incorporated as the Assignee thereof and which reference is incorporated herein, discloses such a damage-tolerant epoxy resin matrix having high-strength filaments therein.
The epoxy resin matrix, usually containing strength-enhancing fibers therein, is laid upon the outer surface of the EPDM rubber material to form an outer shell of a desired thickness, thereby creating a motor case assembly. The motor case assembly, usually including the tooling mandrel to provide support to the motor assembly, which comprises the uncured EPDM rubber material layer and the uncured fiber reinforced epoxy resin matrix layer, is typically placed in an oven or an autoclave and co-cured at an elevated temperature and at a preselected pressure which is usually the ambient, or atmospheric, pressure. As a result of co-curing the rubber material layer and the epoxy resin layer together or, more accurately, vulcanizing the rubber layer and curing the epoxy resin layer simultaneously while the rubber substrate and the epoxy substrate are in contact with each other, a generally suitable high-strength bonding of the two substrates occurs. Upon the assembly being co-cured and thus becoming bonded together, the assembly is removed from the oven, autoclave or other curing apparatus, and the removable mandrel is removed from within the now-bonded rocket motor case assembly.
The previously described rubber materials and aromatic cured epoxy resin based composite materials generally performed adequately together in forming a suitably strong and uniform bond between the rubber substrate and the epoxy resin composite substrate. However, it was discovered when bonding EPDM rubber materials, or compounds, to aliphatic type epoxy resins that the rubber-to-aliphatic epoxy bonds did not have the same bond qualities as did the rubber-to-aromatic epoxy bonds. Aliphatic epoxies are often designed to have enhanced, or at least differing, room temperature handling characteristics when compared with aromatic epoxies. Furthermore, aliphatic epoxies are generally designed to have particularly modified viscosity-related characteristics affecting the ability of fibers to be impregnated with the aliphatic epoxy resin and to have longer shelf lives in which the prepreg material could be stored prior to being used. In order to provide such desirable qualities, aliphatic epoxies have a different chemical composition which appears to significantly negatively affect their ability to be bonded to certain rubber substrates, at least when compared to the bonding of aromatic epoxy based substrates to EPDM rubber substrates. A prepreg fiber composition including such an aliphatic epoxy that is anhydride cured is discussed in U.S. Pat. No. 5,593,770, issued to Mumford et al. and entitled Chemorheologically Tailored Matrix Resin Formulations Containing Anhydride Curing Agents. The ""770 patent is a continuation-in-part of U.S. Pat. No. 5,356,499, issued to Decker et al. and entitled Method For Increasing Fiber Strength Translation In Composite Pressure Vessels Using Matrix Resin Formulations Containing Surface Acting Agents, directed to a method for improving the strength of composite pressure vessels incorporating chemorheologically viscosity tailored epoxy resin formulations having certain viscosity characteristics. Both of these patents are incorporated by reference herein.
U.S. Pat. No. 5,656,703, issued to Costin et al. and entitled Coating Composition of Epoxy Resin, Metal Di(Meth)Acrylate and Poly(Meth)Acrylate, discloses a curable coating composition containing an epoxy resin, a polymethacrylate, and a metal salt to improve adhesive characteristics: of a coating on a substrate, but does not offer any teachings, or suggestions, directed to bonding substrates of differing materials together, such as rubber being bonded to an epoxy resin based substrate.
U.S. Pat. 5,763,629, issued to Fan et al. and entitled Alkoxylated Glycidyl (Meth)Acrylates and Method of Preparation, is directed to a class of monomers containing epoxide functionalities wherein epoxidation of the monomers makes use of an organic solvent such as a hydrocarbon-based solvent, but does not offer any teachings, or suggestions, directed to bonding substrates of differing materials together such as rubber being bonded to an epoxy resin based substrate.
During testing, the bonds between EPDM rubber and aliphatic epoxies in particular seemed to lack sufficient adhesive strength when compared to bonds between the previously mentioned aromatic epoxies and EPDM rubber. For example, upon tensile testing of a test sample, the bond between an EPDM rubber substrate and an aliphatic anhydride-cured epoxy based composite material (which had been co-cured as discussed previously) would suffer an adhesive failure. That is, the test sample would pull apart at the bond line, or at the interface the rubber material and the aliphatic epoxy based composite material instead of preferably cohesively failing wherein either the rubber material or the composite material would be pulled apart from itself. In this particular case, it would be expected that the rubber material, having relatively lower tensile and shear strength ratings, would cohesively fail before the composite matrix material, having relatively higher tensile and shear, strength ratings, if an adhesive failure at the bond line did not occur first.
For a variety of reasons and notwithstanding the above-discussed test results, the use of aliphatic or cylcoaliphatic epoxy resins, generally referred to as aliphatic epoxy resins such as the epoxy resin disclosed in previously referenced U.S. Pat. No. 5,593,770xe2x80x94Mumford et al., offers certain advantages in certain applications over aromatic epoxy resins. Furthermore, the use of aliphatic epoxy resins, or aromatic epoxies having poor bonding characteristics, may be required by the product specifications issued in connection with a contract, thus leaving few choices for the contracting party making a given product other than to perhaps seek approval or recertification for bond-enhancing additives to be included in the specified rubber material and/or be included in the specified aliphatic epoxy resin based matrix, or both, to enhance the bond therebetween. However, as a practical matter it would be time consuming and costly for the contracting party to identify and then seek approval and re-certification of either or both substrates to be formulated to include such bond-enhancing additives to achieve acceptable bond strengths.
One alternative considered by the inventor of the present invention included cold bonding the vulcanized rubber material to the cured aliphatic epoxy resin matrix. However, this particular alternative introduces several additional processing steps leading to increased complexity and manufacturing costs. Moreover, when cold bonding it is often difficult to reliably obtain a sufficiently strong bond when bonding substrates having low surface energies. This problem is partly due to it not always being possible to ensure full-contact interfaces, wherein an adhesive can fully and uniformly be applied between the rubber material and the epoxy resin matrix.
Another alternative considered by the inventor of the present invention was the use of commercially available adhesives such as Chemlock 234 and Chemlock 236 available from the Lord Corporation. But these adhesives, as with most, high-strength, high-temperature resistant adhesives in general, usually contain certain solvents or blends of solvents requiring special handling techniques and precautions to ensure safety and compliance with environmental restrictions.
Another problem identified with using known adhesives is that such adhesives are often formulated to bond a number of different types of substrates together and hence may have a number of active ingredients. For example, a candidate adhesive might be formulated for bonding metals, ceramics, glass, and plastics, etc., and thus have a plurality of active ingredients, wherein each active ingredient is particularly suitable for bonding a particular material. However, when using such adhesives having multiple active ingredients, there is a concern as to what effect each of the active ingredients will have on the bond strength. That is, one or more of the extraneous active ingredients may actually decrease the bond strength of the bond provided by the particularly suitable active ingredient with respect to the base materials being bonded. Furthermore, when using adhesives having multiple active ingredients, there is always a concern as to potential or actual adverse effects each of the multiple active ingredients, whether acting individually or in combination with one or more of the other ingredients, could have on the structural, durability, longevity, performance, and compatibility properties of the base materials to be bonded.
Upon investigations, and in some cases tests, the preceding alternatives were deemed unattractive or not acceptable. For example, inferior bond strengths were evidenced by test samples failing adhesively instead of cohesively, i.e., at the bond interface between two substrates instead of one of the substrates being pulled apart from itself. Other factors making the above alternatives unattractive or unacceptable included associated increased material costs and/or manufacturing costs, potential adverse environmental impacts, and concerns as to whether a readily available adhesive being applied between the two materials would significantly alter the properties of one or both of the materials. If a significant property change of either substrate were at all suspected, substantial product testing would have to be conducted to ensure that product durability, performance and life expectancies were not negatively affected.
Thus, it can be appreciated that there is a need within the art for a method of bonding rubber materials to epoxy resin materials wherein the resulting bond is consistently durable, reliable, of a suitable quality, and in which manufacturing costs and potentially adverse environmental factors associated with such a method are kept to a minimum.
It can be further appreciated that there is a need within the art for a method of bonding rubber materials to epoxy resin materials in which there are no substantial changes resulting to the properties of the two materials to be bonded together.
Another need within the Art is for a method of bonding rubber materials to epoxy resin materials to form selected components of rocket motors without unduly adding to the cost of manufacturing such motors by having to have certain solvents on hand which typically require special handling techniques when using such solvents and which typically require certain precautions be taken when storing and disposing such solvents.
A yet further need within the art is the ability to bond an unvulcanized synthetic rubber substrate to an aliphatic epoxy resin based composite substrate reliably and consistently, which is known to be particularly difficult to achieve.
A still yet further need within the art is the ability to bond rubber materials to epoxy resin based composite materials having reinforcing fibrous elements therein, such as carbon fibers, graphite fibers, and boron fibers, aramid fibers, typically provided in the form of tapes, woven fabrics, mats, tows, and windable filaments, for example.
The present invention includes a method of constructing certain components of rocket motors such as thermally insulated composite motor cases. Preferably, the case is constructed of an outer, or exterior, shell formed of an aliphatic epoxy resin based, carbon fiber reinforced composite material in which a heat resistant EPDM rubber based material, or compound, is bonded to an inner surface of the outer composite shell via an interfacial layer. Preferably, the bonding is achieved by vulcanizing the rubber material and curing the epoxy resin based material simultaneously at an elevated temperature and at a preselected pressure. The present invention further provides a bonding compound preferably including at least one rhono-functional or poly-functional acrylate such as glycidyl methacrylate, trimethylolpropane trimethacrylate, and pentaerythritol triacrylate, mixed with a selected amount of EPDM rubber material, or compound. Such EPDM rubber material, or compound, could be taken from the same batch used in forming the rubber substrate to be bonded, or the EPDM rubber material could be formulated to the same specifications as the rubber substrate to be bonded. Alternatively, the rubber constituent of the bonding compound could merely be a rubber material or rubber compound that is the same or similar to the rubber or rubber compound used in forming the substrate to be bonded. An optional solvent, preferably comprising at least one hydrocarbon such as a terpene, to facilitate mixing the bonding compound and for disposing the compound on one or more of the substrates to be bonded to form an interfacial layer therebetween, may be added to the compound. The optional solvent evaporates after application, thereby providing an interfacial layer of dry residual material. Preferably, the percentage of rubber and percentage of acrylate on a weight basis ranges from approximately 95% rubber and 5% acrylate to approximately 5% rubber and 95% acrylate, with a percentage of approximately 40% rubber and a percentage of approximately 60% acrylate, producing a bonding compound which yields very favorable results. However, higher and lower concentrations of rubber and acrylate, each ranging from 0% to 100% respectively, with a trace of acrylate, with or without the use of a solvent, may be used in Accordance with the present invention.
Although the present invention is particularly suitable for bonding an unvulcanized EPDM rubber substrate to an aliphatic anhydride-cured, epoxy resin based composite substrate having carbon fibers therein to form a rocket motor case, it is not limited to such. The introduction of an interfacial layer containing at least one acrylate can be used in the bonding of a wide variety of rubber substrates to a wide variety of epoxy resin based substrates to form a wide variety of products, articles, or assemblies. For example, the rubber substrate to be bonded may be a natural rubber, a synthetic rubber, or it may be unvulcanized, or it may have been previously vulcanized. Likewise, the epoxy resin based substrate can be an aromatic epoxy resin and need not be an aliphatic epoxy resin.